Hot wire air flow meter

ABSTRACT

An exothermic resistor for use in a hot wire air flow meter, having a wire (2) in the form of a coil made of a metal, a pair of lead wires (3) connected to connections (21) formed at the opposite ends of the coil, and a support member (4) which integrally supports the connections as well as the coil located between the connections. An example of the support member is in the form of a tubular cylinder closed at its both ends. The support member is formed from glass only or formed of a layer of a glass-ceramic-composite material. The hot wire air flow meter is designed to be easely mass-produced and to have improved transient response characteristics with respect to abrupt changes in the air flow rate.

This application is a continuation of application Ser. No. 643,482,filed on Jan. 22, 1991, now abandoned, which is a divisional ofapplication Ser. No. 250,212, filed Sept. 28, 1988, now U.S. Pat. No.5,020,214.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a hot wire air flow meter. More particularly,this invention relates to a hot wire air flow meter suitable formeasurement of the flow rate of air taken into an internal combustionengine for a motor vehicle, and also relates to a method ofmanufacturing such a hot wire air flow meter.

2. Description of the Prior Art

A hot wire air flow meter has a heating coil which is formed as anexothermic resistor that is disposed in an air flow path the flow rateof which is to be measured. In order to eliminate any reduction in thetemperature of the heating coil due to the cooling effect of the airflow, a current which flows through the air flow path is increased so asto heat up the coil. The air flow rate is determined from this increasein the current. This type of air flow meter can be constituted withoutemploying any movable parts and, at the same time, it enables directdetection of the mass flow. For this reason, air flow meters of thistype are generally adopted to perform air-fuel ratio control in theinternal combustion engines of motor vehicles.

The exothermic resistor provided in this type of air flow metercomprises a very thin metal wire, e.g., a platinum wire having adiameter of several tens microns. For instance, an exothermic resistorsuch as the one disclosed in Japanese Utility Model Laid-Open No.56-96326/1981 is formed in such a manner that a metal wire provided asan exothermic resistance wire is wound around a core wire, that is, abobbin made of a ceramic material.

Another type of exothermic resistor is disclosed in an already filedpatent application (now U.S. Pat. No. 4,790,182) which is a bobbinlessexothermic resistor in the form of a coil formed from a metal wire andovercoated with glass except for opposite end portions which are weldedto a support for the exothermic resistor.

In the case of an exothermic resistor formed of a metal wire woundaround a core wire or bobbin made of a ceramic material employing one ofthe above conventional techniques, the quantity of heat heating up thebody of the bobbin and the quantity of heat transmitted through thebobbin to the support on which the exothermic resistor rests are notnegligible. There is therefore a problem of retardation of the transientresponse to any fluctuation in the air flow, in particular, resulting inthe occurrence of surging when the vehicle is sharply accelerated ordecelerated. In addition, it is necessary during the process ofmanufacturing exothermic resistors to perform a coil winding operationfor each exothermic resistor, which makes automatization of the processdifficult.

In contrast, the bobbinless type of exothermic resistor has improvedresponse characteristics and can be manufactured with an improved degreeof automatization because the coil winding operation can be continuouslyperformed for a plurality of resistors of this type. However, it isdifficult to handle the opposite end portions of the wire which are notcoated with glass, and there is a problem regarding a reduction in theease with which the operation of securing the resistor to the supportcan be performed in the manufacturing process. In addition, the layer ofcoating glass which acts as a support member for supporting the coiledportion of the wire must have a substantial thickness so as to ensurethe specified strength of the final products. In consequence, heattransfer between the wire and the air flow is obstructed due to theglass layer having inferior heat conductivity, thereby causingdeterioration in the transient response characteristics.

In the structure of the bobbinless type of exothermic resistor, theinner surface of the cylindrical member which is formed by means ofglass coating (in which the wire extends helically) is brought intocontact with the outside air. If any dust and/or ionic substances arecontained in the air the flow rate of which is to be measured, the dustand/or ionic substances become attached to the inner surface of thecylindrical member, or, in the worst case, the inner space of thecylindrical member becomes filled with accumulated dust. In this worstcase, the heat generating from the wire is transmitted through themedium of the dust, thereby impairing the advantage of the bobbinlesstype. If the inner space of the resistor is filled with attached andaccumulated ionic substances, short circuiting takes place betweenadjacent coiled portions of the wire, and characteristics specific tothe exothermic resistor are thereby changed. In a method ofmanufacturing the conventional bobbinless exothermic resistor, a step ofremoving a bobbin after a coil has been formed by winding a metal wirearound this bobbin is adopted, and chemical etching is utilized as ameans for removing the bobbin, thereby necessitating an additionalprocess for performing this etching. This makes the overallmanufacturing process more complicated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a hot wire air flowmeter having an exothermic resistor and a method of manufacturing thesame, the hot wire flow meter being capable of being manufactured withan increased degree of automatization as well as being easily handledand having suitable response characteristics.

It is another object of the present invention to provide a hot wire airflow meter having an exothermic resistor, the exothermic resistor beingcapable of maintaining suitable response characteristics in the face ofthe influence of dust or ionic substances contained in the air as wellas being protected against any deterioration in its overallcharacteristics.

It is still another object of the present invention to provide a methodof manufacturing a hot wire air flow meter in which a complicated steppreviously forming part of the manufacturing process is eliminated.

To these ends, the present invention provides a hot wire air flow meterhaving an exothermic resistor having a metal wire or an exothermicresistance wire wound in a coil, metal lead wires connected to oppositeends of this coil, and a support member made of, for example, a glass,the support member supporting (integrally fixing) connections betweenthe wound wire and the lead wires as well as the wound wire locatedbetween these connections.

This type of hot wire air flow meter is manufactured by a methodincluding: preparing a blank constituted by a metal core wire having adesired length, a metal wire wound around the core wire and used as anexothermic resistance wire, and a pair of lead wires welded to oppositeends of the wound wire; overcoating the wound wire with a glass materialsuch that this glass material also covers welded portions; sintering theovercoating; and thereafter removing the core wire. If a glass core isused in place of the core wire, there is no need for the step ofremoving the core.

In accordance with the present invention, a metal wire is continuouslywound around a metal core wire or a glass core line to form a lengthwiseblank having a length corresponding to a plurality of exothermicresistors successively disposed in line. At this time, the metal wire iscontinuously wound at a time by an automatic winding machine for theplurality of exothermic resistors, thereby remarkably increasing thedegree of automatization of the manufacturing process.

Thereafter, the thus-prepared blank constituted by the core wire and theresistance wire is cut into pieces, each having a desired length, andlead wires are welded to opposite ends of each piece. Welded portions,as well as the resistance wire located between the welded portions, areovercoated with a glass so that they are fixed integrally. Inconsequence, the exothermic resistance wire is supported (integrallyfixed) by the lead wires and the glass member, and this exothermicresistor is easy to handle since there is no need for an operation ofconnecting thin wires to the support for the exothermic resistor.

The metal core wire short-circuits the coil between the lead wires sinceit has electro-conductive properties. It is therefore removed by meansof, for example, etching using an acid. It is possible to improve theexothermic portion in terms of mechanical strength and corrosionresistance by the overcoating glass. It is thereby ensured that the heatcaused by energization of the resistance wire can be almost entirelytransmitted to the air without heating the bobbin or core having a largeheat capacity or being transmitted to the support via the bobbin as inthe case of the conventional bobbin-type exothermic resistor. It istherefore possible for the hot wire air flow meter to be improved in theresponse characteristics with respect to abrupt changes in the air flowrate and to output signals by suitably following up actual changes inthe air flow rate, thereby optimizing the fuel supply control andsolving the problem of surging, etc.

If the glass core line is used, there is no need for the step ofremoving the core since the glass core is non-electro-conductive. Inthis case, it is possible to reduce the thickness of the glassovercoating so as to avoid any considerable increase in the total heatcapacity because the mechanical strength of the exothermic resistor canbe maintained by the core. As a result, the response characteristics donot become inferior compared with the case where the metal core wire isremoved.

A method of filling the cavity formed by removing the metal core wirewith a glass is also possible. The resulting resistor has the samecharacteristics as the resistor having the glass core.

The resistance wire is ordinarily formed of a platinum wire superior interms of thermal resistance and corrosion resistance. Instead, it can beformed of a tungsten wire. The lead wires may be wires made of aplatinum-iridium.

If the glass is baked at an excessively high temperature, the platinumwire becomes embrittled and the electrical characteristics thereofbecome changed. Heating at a temperature higher than about 1200° C. fora long time must be avoided. Correspondingly, to perform baking at atemperature lower than 1200° C., a glass material having a viscosity of10⁴ to 10⁷ poise at temperatures of 800° to 850° C. is used as the glassmaterial for supporting the exothermic resistor. It is not alwaysnecessary that the thermal expansion coefficient of the glass is equalto that of the platinum wire (90×10⁻⁷ /° C.). However, this is preferredin terms of reduction in the stress due to the heat cycle duringoperation. If the core wire is removed by etching using an acid, it isnecessary to prevent the glass from becoming greatly eroded. The acidresistance of the glass, as well as the viscosity properties, stronglyrelates to the connective strength of the structure of the glass. It wasconfirmed that, in the case of a glass having this viscosity-strength,erosion was limited to a depth of not greater than 1 μm. This glass isalso suitable in terms of water resistance and oil resistance in anapplied state. A glass the viscosity of which becomes reduced at a lowertemperature can be baked at a lower temperature but the acid, water andoil resistances of this glass are insufficient.

A glass having the above-described suitable properties can be selectedfrom lead-potash glass, lead-soda glass, lead-potash-soda glass,soda-lime glass, soda-barium glass, potash-lime glass, potash-bariumglass and borosilicate glass.

A molybdenum wire or nickel-iron alloy wire is used as the metal corewire. With respect to these materials, heating at a temperature higherthan 1200° C. in the atmosphere is not preferable. They can be used incombination with a glass having the above-described properties.Specifically, the thermal expansion coefficient of the nickel-iron alloywire can be adjusted to that of the platinum wire, enabling a reductionin the thermal stress at the time of baking of the glass.

To attain the above objects, the present invention provides a hot wireair flow meter comprising an exothermic resistor including a glassmember in the form of a cylindrical tube provided as a support member,and a metal wire in the form of a coil extending helically along theinner surface of the glass member and coaxially therewith and having itsboth ends electrically led to the outside of the glass member, whereinthe glass member is closed at its both ends.

This exothermic resistor is manufactured by a method including steps of:winding a metal wire as an exothermic resistance wire around asublimatable core wire; covering the wound wire and the core wire with aporous glass material; and performing heat treatment to sublimate thecore wire and bake the glass.

The above cylindrical glass member having the inner surface along whichthe coil is formed is closed at its both ends, thereby preventing dustor ionic substances in the air from attaching the inner surface of theglass member or filling the cavity thereof. There is no possibility ofany extraneous substance reaching the surface along which the coil isformed. The properties of the resistor is stabilized in terms of thermalor electrical effects, thereby attaining the above object.

In the manufacturing process, a sublimatable material is used for thecore wire, and the core wire is sublimated at the time of baking of theglass by the heat required for this baking. It is therefore possible toeliminate the need for the etching step with respect to the removal ofthe core wire, thereby simplifying the process.

To attain the above objects, the present invention also provides an airflow meter comprising an exothermic resistor having an exothermicresistance wire or metal wire, a pair of lead wires connected to thewire, and a support member covering and supporting the exothermicresistance wire, wherein the support member is formed of a layer of acomposite material composed of ceramic and glass materials.

The performance of this type of exothermic resistor can be effectivelyimproved if this resistor has a cavity and if the glass component of thecomposite material forming the support member forms a surface layerthereof and also forms a continuous phase which reaches the cavity.

A ceramic material having a thermal conductivity of at least 10 W/m·Kmay be selected as the ceramic component of the composite materiallayer, which is also effective. It is preferable to set the proportionof the glass component of the composite material layer to 2 to 60 volumepercent. The glass component of the composite material layer may includea glass having a softening temperature not higher than 700° C. andanother glass having a softening temperature higher than 700° C., whichis effective.

The exothermic resistor may have another type of structure in which theexothermic resistance wire is formed of a film circuit formed on aceramic substrate while the support member is formed of the ceramicsubstrate and a layer of a composite material composed of ceramics andglass.

To attain the above objects, the present invention also provides amethod of manufacturing an air flow meter, including steps of: preparinga blank constituted by a metal core wire having a desired length, ametal wire wound around the core wire and used as an exothermicresistance wire, and a pair of lead wires connected to opposite ends ofthe wound wire; depositing ceramic particles to the wound wire over theentire surface thereof and thereafter sintering the ceramic particles;removing the core wire; and forming a layer of composite material bycoating the layer of sintered ceramic material with molten glass so thatthe glass permeates into the layer of sintered ceramic material. Withrespect to the step of preparing a blank constituted by a metal corewire, a metal wire wound around the core wire, and lead wires, it ispossible to adopt a method of continuously winding a metal wire used asan exothermic resistance wire around a long metal core wire, cutting ablank thereby formed into pieces each having the desired length, andthereafter connecting the led wires to the opposite ends of the woundmetal wire, or a method of connecting the lead wires to opposite ends ofthe metal core wire (having the desired length), connecting one end ofthe metal wire used as an exothermic resistor to one of the lead wires,winding this resistance wire around the core wire, and thereafterconnecting the other end to the other lead wire.

The layer of composite material composed of ceramics and glass may beformed by coating a mixed ceramic and glass particles to the wound metalwire over the entire area thereof and thereafter sintering thismaterial. A glass coating may be formed over the composite materiallayer in a molten glass coating manner. Instead of this method of mixingceramic and glass particles, it is possible to adopt a method of formingparticles from a composite material composed of ceramics and glass,coating the metal wire wound around the metal core wire with theseparticles, and thereafter sintering this material.

The present invention also provides an air flow meter of an motorvehicle having the above-described exothermic resistor and a drivingcircuit which controls the current flowing through the exothermicresistor and takes out the voltage output from the exothermic resistoras a signal corresponding to the air flow rate. It also provides ananemometer having the above-described exothermic resistor and a meansfor detecting the temperature of the exothermic resistor.

In accordance with the present invention, the opposite ends of theresistance wire are connected to the lead wires, and therefore there inno need for an operation of connecting thin metal wires to the support,thereby making the resulting resistor easy to handle. In particular, theresistance wire, as well as the connections between the wire and thelead wires, is ordinarily covered with the support member so that theresistance wire and the lead wires are fixed. In consequence, theexothermic resistance wire is supported by the lead wires and thesupport member, thus realizing a structure suitable for handling of theresistor.

The core wire short-circuits the coil between the lead wires since ithas electro-conductive properties. It is therefore removed by, forexample, etching using an acid, or oxidation and sublimation at anincreased temperature in the atmospheric air. The exothermic portion canbe improved in the mechanical strength and the resistance toenvironmental influences by virtue of the composite material coating.The heat generated by the electrical current through the resistance wireis transmitted to the air via the composite material layer. It ispossible to set the thermal conductivity of this composite materiallayer to at least about ten times as high as that of a glass, which isabout 1 W/m·K, if this composite material layer is composed of thisglass and a ceramic material having a thermal conductivity of at least10 W/m·K. It is therefore possible to avoid a considerable retardationof the change in the heat transfer rate in response to a change in theair flow rate, thereby improving the transient response characteristics.

If the coating layer formed on the resistance wire is baked at anexcessively high temperature, the platinum wire becomes embrittled andthe electrical characteristics thereof become changed. Heating at atemperature higher than about 1200° C. for a long time must be avoided.For this reason, if the wire is coated with only a ceramic materialhaving a high thermal conductivity, baking is not suitably performed andthe strength of the exothermic resistor becomes inadequate. If thecomposite material layer is formed by coating the baked ceramic layerwith a molten glass so that the glass permeates into this layer, it hasan adequate strength as well as a high degree of thermal conductivity.If ceramic and glass materials are simultaneously coated to theresistance wire and are thereafter sintered, a composite materialcoating layer having an improved strength can be obtained by thesintering effect of the glass even if the composite material is sinteredat a temperature not higher than 1200° C. In the case where there arepores remaining in the composite material layer, the layer is furthercoated with a molten glass to obtain a higher strength.

The uniformity of the components of the composite material layer can beimproved if particles formed from a composite material composed ofceramic and glass materials are attached as a coating to the resistancewire and are thereafter baked. It is thereby possible to improve theaccuracy with which the compounding ratio control is performed.

In terms of ease of coating, it is preferable to use, as the coatingglass, a glass having a softening temperature lower than 700° C.

If the glass component of the composite material layer forms acontinuous phase, the strength of the support member is increased. Ifthe proportion of the glass component is excessively small, the strengthof the support member becomes inadequate and, if the proportion of theglass component is excessively large, the effect of the compositionmaking use of the ceramic material in order to increase the thermalconductivity is reduced. A suitable range of the proportion of the glasscomponent of the composite material layer is 2 to 60 volume percent.

A molybdenum wire of a nickel-iron alloy used as the metal core wire canbe removed by etching using an acid. The molybdenum wire can be removedduring sintering of the coating layer since it sublimates by oxidizingin the air.

A type of air flow sensor element, that is, an exothermic resistorhaving an exothermic circuit constituted by a film pattern formed on analumina substrate, and a support member which covers this film circuitand the alumina substrate has a reduced thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of an exothermic resistorfor use in a hot wire air flow meter of the present invention;

FIGS. 2A to 2E are diagrams of the process of manufacturing theexothermic resistor shown in FIG. 1;

FIG. 3 is a longitudinal cross-sectional view of an exothermic resistorfor use in another embodiment of the present invention;

FIGS. 4A to 4D are diagrams of the process of manufacturing theexothermic resistor shown in FIG. 3;

FIG. 4E is an enlarged illustration of a portion of the exothermicresistor shown in FIG. 4D;

FIG. 5 is an illustration of another example of the method ofmanufacturing the exothermic resistor in accordance with the presentinvention;

FIG. 6 is a longitudinal cross-sectional view of an exothermic resistorfor use in still another embodiment of the present invention;

FIGS. 7A to 7E are diagrams of the process of manufacturing theexothermic resistor shown in FIG. 6;

FIG. 8 is a cross-sectional view of a state in which ceramic and glassmaterials are attached to an exothermic resistance wire wound around acore wire;

FIG. 9 is a graph of the relationship between the volume percent of theglass component and the anti-crushing strength;

FIG. 10 is a graph of the relationship between the volume percent of theglass component and the response time;

FIGS. 11A to 11D are diagram of another example of the method ofmanufacturing the exothermic resistor shown in FIG. 6;

FIGS. 12A to 12D are cross-sectional views of the process ofmanufacturing an exothermic resistor with a film circuit;

FIG. 13 is a perspective view of an exothermic resistor with a filmcircuit for use in a further embodiment of the present invention;

FIG. 14 is a cross-sectional view of a hot wire air flow meter;

FIG. 15 is a circuit diagram of a driving circuit of the hot wire airflow meter shown in FIG. 14; and

FIG. 16 is a graph of the response characteristics of the hot wire airflow meter shown in FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 shows the structure of an exothermic resistor 1 for use in afirst embodiment of the present invention. The exothermic resistor 1 fordetecting the intake air flow rate has a length of platinum wire 2 inthe form of a coil, a pair of lead wires 3 made of a platinum-iridiumalloy connected to opposite ends of the platinum wire coil 2, and aglass member 4 which supports the platinum wire 2 and connections 21between the lead wires 3 and the platinum wire 2.

A method of manufacturing the exothermic resistor 1 will be describedbelow with reference to FIGS. 2A to 2E.

FIG. 2A shows a core wire or a bobbin 5 which has a diameter of 0.5 mmand is made of a Ni-Fe alloy having a Ni content of 52% and around whichthe platinum wire 2 is to be wound. As shown in FIG. 2B, a length ofplatinum wire 2 having a diameter of 20 μm was continuously wound aroundthe core wire 5 over a length corresponding to two or more exothermicresistors by employing an automatic coil winding machine. As shown inFIG. 2C, a resistor blank thereby formed was cut into pieces, eachhaving a length of 6 mm, and a pair of lead wires 3 made of aplatinum-iridium alloy and having a diameter of 0.13 mm were welded toopposite ends of each piece at connections 21. Then, as shown in FIG.2D, the connections 21 and the platinum wire 2 were overcoated with aglass material 4 in order to form a support member, and thereafterunderwent sintering. The glass material 4 was a lead-potash-soda glasshaving a composition consisting of, by weight, 56% of SiO₂, 30% of PbO,6% of K₂ O, 6% of Na₂ O, 1% of CaO, and 1% of Al₂ O₃. The viscosity ofthis glass was measured and found to be 10⁶.3 poise at 800° C. and 10⁶poise at 850° C. To perform overcoating, a solution was prepared whichcontained denatured alcohol and water as solvents and magnesium nitrateand aluminum nitrate provided as electrolytes, and in which the aboveglass material was dispersed; and glass powder was attached to theplatinum wire by electrophoresis in such a manner that the platinum wirein the state shown in FIG. 2C was placed in the suspension forelectrophoretic deposition as a cathode while an aluminum plate wasplaced in the same suspension as an anode, a voltage of 30 V then beingapplied through the electrodeposition liquid between these electrodes.Thereafter, the thus-processed piece was sintered in an electric furnaceat 800° C. for 6 minutes. The thickness of the glass was about 100 μm.FIG. 2E shows a state in which the core wire has been removed byimmersing the sintered piece in a mixed acid composed of nitric acid andsulfuric acid at 80° C. for 1 hour. The glass became eroded by the mixedacid to a depth of 1 μm or less. The exothermic resistor 1 in the stateshown in FIG. 2E has a strength sufficient enough to resist being brokenwhen it is handled by a pincette, and it can be treated as a singleelement. It is therefore easy to handle in the succeeding assemblyprocess, thus improving the ease with which operations relating tohandling of the exothermic resistor can be performed.

Embodiment 2

Exothermic resistors of the structure shown in FIG. 1 were manufacturedin the same manner as Embodiment 1 by using various types of glass.

Table 1 shows the compositions of glasses used. Table 2 shows theviscosity of each glass measured at 800° C. and 850° C., the temperatureat which each piece was sintered, whether or not the platinum wire wasembrittled, and whether or not each exothermic resistor was broken whenit was handled by a pincette after the core wire had been removed. Astate in which the glass became eroded during etching of the core wiresuch that the platinum wire was exposed and was partly unwound isincluded in the kinds of breakage to be put in the table.

As can be understood from Table 2, it is necessary for a glass having aviscosity higher than 10⁷ poise at 800° C. to be sintered at atemperature higher than 1200° C. Sintering at this temperatureembrittles the platinum wire. A glass having a viscosity lower than 10⁴poise may be sintered at a lower temperature, but it tends to erodeduring etching and is inferior in terms of strength.

Of the exothermic resistors listed in FIG. 2, each of those free fromembrittlement of the platinum wire and breakages (those making use ofglasses b, e, f, h, and i) was used to constitute a hot wire air flowmeter shown in FIG. 14. It was proved that, as shown in FIG. 16,response characteristics of hot wire air flow meters thereby made weresuperior than those of an air flow meter making use of the conventionalbobbin-type exothermic resistor.

                  TABLE 1                                                         ______________________________________                                        Glass Composition (percent by weight)                                         ______________________________________                                        a     SiO.sub.2 35,                                                                          PbO 58,  K.sub.2 O  7                                          b     SiO.sub.2 50,                                                                          PbO 35,  K.sub.2 O  5,                                                                         Na.sub.2 O  8,                                                                       Al.sub.2 O.sub.3  2                    c     SiO.sub.2 70,                                                                          PbO 12,  K.sub.2 O  6,                                                                         Na.sub.2 O  7,                                                                       CaO  5                                 d     SiO.sub.2 60,                                                                          BaO 12,  MgO  5, Na.sub.2 O 12,                                                                       Al.sub.2 O.sub.3 11                    e     SiO.sub.2 65,                                                                          CaO  2,  BaO 13, K.sub.2 O 15,                                                                        Al.sub.2 O.sub.3  5                    f     SiO.sub.2 72,                                                                          CaO  4,  MgO  3, Na.sub.2 O 20,                                                                       Al.sub.2 O.sub.3  1                    g     SiO.sub.2 53,                                                                          B.sub.2 O.sub.3  9,                                                                    Al.sub.2 O.sub.3 20,                                                                  CaO, 15,                                                                             BaO  3                                 h     SiO.sub.2 65,                                                                          B.sub.2 O.sub.3 18,                                                                    Al.sub.2 O.sub.3  7,                                                                  Na.sub.2 O  6,                                                                       BaO  4                                 i     SiO.sub.2 70,                                                                          B.sub.2 O.sub.3 16,                                                                    Al.sub.2 O.sub. 3  4,                                                                 K.sub.2 O  4,                                                                        PbO  6                                 j     SiO.sub.2 80,                                                                          B.sub.2 O.sub.3 13,                                                                    Al.sub.2 O.sub.3  2,                                                                  Na.sub.2 O  4,                                                                       CaO  1                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                      Embrittle-                                      Types              Sintering  ment of                                         of    Viscosity (poise)                                                                          temperature                                                                              platinum                                        glass 800° C.                                                                         850° C.                                                                        (°C.)                                                                           wire    Breakage                              ______________________________________                                        a     10.sup.4 10.sup.3                                                                              690      No      Yes                                   b     10.sup.5.2                                                                             10.sup.4.7                                                                            730      No      No                                    c     10.sup.7.3                                                                             10.sup.6.6                                                                            1210     Yes     No                                    d     10.sup.4.6                                                                             10.sup.3.7                                                                            560      No      Yes                                   e     10.sup.5.5                                                                             10.sup.5                                                                              760      No      No                                    f     10.sup.6 10.sup.5.5                                                                            800      No      No                                    g     10.sup.9 10.sup.8.4                                                                            1250     Yes     No                                    h     10.sup.6.5                                                                             10.sup.5.8                                                                            760      No      No                                    i     10.sup.6.9                                                                             10.sup.6.1                                                                            860      No      No                                    j     10.sup.7.8                                                                             10.sup.6.9                                                                            1220     Yes     No                                    ______________________________________                                    

Embodiment 3

A molybdenum wire having a diameter of 0.5 mm was used as the core wire5. A platinum wire was wound around this core wire, a resistor blankthereby formed was cut into pieces and a pair of lead wires were weldedto each piece in the same manner as the manufacturing process shown inFIGS. 2A to 2C. Glass was attached to the welded portions and to theplatinum wire by a dipping method, the composition of this glassconsisting of, by weight, 74% of SiO₂, 9% of CaO, 8% of K₂ O, 8% of Na₂O, and 1% of Al₂ O₃. The viscosity of the glass was 10⁶.2 poise at 800°C. and 10⁵.5 at 850° C. The blank piece was thereafter heated in theelectric furnace at 1000° C. for 30 minutes, thereby sintering theglass. In this case, the ambient atmosphere inside the electric furnacewas atmospheric air, and the molybdenum core wire was removed duringsintering by being oxidized and sublimated. An exothermic resistorsimilar to that shown in FIG. 2E was thus obtained.

The thus-obtained exothermic resistor had a work strength enough for thesucceeding assembly process and a hot wire air flow meter making use ofthis exothermic resistor exhibited improved response characteristics, asshown in FIG. 16.

As can be understood from the foregoing, the present invention iseffective irrespective of the method of coating glass and the method ofremoving the core wire.

Embodiment 4

Glass fiber filaments having a diameter of 10 μm were bundled to form aglass line having a diameter of 0.3 mm. This glass line was used as acore wire, and a platinum wire was wound around this core wire. Aresistor blank thereby formed was cut into pieces, each having a lengthof 10 mm. Lead wires were welded to opposite ends of each piece. Thesame glass as that used in Embodiment 1 was attached to each piece byelectrophoresis in such a manner that the core and the platinum wire,including the welded portions, were covered with the glass over theentire area thereof. The thickness of the glass was 2/5 of the thicknessof the glass of Embodiment 1. Thus-formed piece was heated at 900° C.for ten minutes, thereby sintering the glass.

An exothermic resistor thereby obtained had a strength greater than thatof the exothermic resistor of Embodiment 1 and was not broken whendropped from a level of 1 m high to the floor. The rising rate ofresponse characteristics was slightly higher than that in the case ofEmbodiment 3, but it was reduced when the flow rate was higher thanabout 100 kg/h and it was substantially equal to that in the case of thethird embodiment with respect to a range of response time after 30 ms.

Embodiment 5

An exothermic resistor of the type shown in FIG. 2E was manufactured. Inthis case, the thickness of glass coated and sintered was set to a halfof that of the first embodiment. Another glass having a compositionconsisting of, by weight, 35% of SiO₂, 58% of PbO, and 7% of K₂ O wasdispersed in an organic solvent, and a cavity of the blank piece formedby removing the core wire was filled with this dissolved glass. Theblank piece with the glass was heated and sintered in the electricfurnace at 650° C. for ten minutes.

The second glass filling the cavity and subjected to baking had manypores. However, the obtained exothermic resistor exhibited a strengthsubstantially equal to that of the exothermic resistor of the firstembodiment, and it also exhibited response characteristics similar tothose of the third embodiment.

Embodiment 6

FIG. 3 shows the structure of an exothermic resistor for a hot wire airflow meter which represents a further embodiment of the presentinvention.

A glass member 4 in the form of a cylindrical tube shown in FIG. 3 isprovided with a length of platinum wire 2 which extends helically in theinner surface of the cylindrical glass member such that the coil therebyformed is coaxial with the glass member 4. Opposite ends of thisplatinum wire coil 2 are connected to a pair of lead wires 3 supportedon the glass member 4 so that the coil is electrically led to theoutside of the glass member 4. The glass member 4 is closed at its bothends by the same material as itself.

Embodiment 7

A method of manufacturing an exothermic resistor of this structure willbe described below with reference to FIGS. 4A to 4D.

As shown in FIG. 4A, a length of platinum wire 2 having a diameter of 20μm is continuously wound by an automatic winding machine around the corewire 5 made of molybdenum and having a diameter of 0.4 mm. As shown inFIG. 4B, the resistor blank shown in FIG. 4A is cut into pieces, eachhaving a length of about 6 mm for one exothermic resistor, and a pair oflead wires 3 having a diameter of 0.13 mm and made of a platinum-iridiumalloy are welded to opposite ends of each piece at connections 21. Then,as shown in FIG. 4C, a glass material 41 is applied, by electrophoresis,over outer surfaces of each piece formed by cutting from the molybdenumcore wire 5 and the platinum wire 2 wound around the core wire and isbaked in an oxidizing atmosphere. The glass material 41 is, for example,a SiO₂ --B₂ O₃ --PbO glass having a viscosity of 10⁶.5 poise at 800° C.and a viscosity of 10⁴.2 at 850° C. As the temperature in the sinteringprocess of the glass material 41 rises, oxidation of the molybdenum corewire 5 is promoted so that the core material becomes MoO₃. When heatedat 795° C., MoO₃ is sublimated while the glass material 41 having aviscosity of 10⁶.5 at 800° C. maintains sufficient open pores, so thatsublimated MoO₃ is dispersed through the open pores between particles ofthe glass material 41, thus removing the molybdenum core wire 5.Thereafter, the temperature was held at 950° C. for 20 minutes, therebycompleting sintering of the glass material. During this process, theglass material 41 reacts with sublimated MoO₃ and the fluidity of theglass is thereby reduced so that the glass becomes porous and thesmoothness of the glass surface becomes inadequate. For this reason, asecondary layer of glass material 42 is formed over the surface of theglass 41, and is baked in an oxidizing atmosphere by the electricfurnace. The glass material 42 is, for example, a ZnO--B₂ O₃ --SiO₂glass having a viscosity of 10⁸ poise at 600° C. and a viscosity of 10⁴poise at 690° C. During sintering at 720° C. for 20 minutes, the glassmaterial 42 adequately fills pores of the porous primary layer of glass41 and forms a smooth outer surface, thereby completing the exothermicresistor 1 shown in FIG. 3. In the case of the exothermic resistorobtained by this method, that is, by being covered with the secondaryglass layer 42 after the molybdenum core wire has been removed and bybeing thereafter sintered, the glass layer extends even inside the coilformed by the platinum wire 2, as shown in FIG. 4E by being enlarged, sothat the platinum wire 2 can be supported more securely.

In the thus-constructed exothermic resistor, the cylindrical glassmember having the inner surface in which the coil is formed is closed atits both ends. Therefore there is no possibility of dust or ionicsubstances becoming attached to the inner surface of the glass member orfilling the inner space of the glass member. In consequence, there is nopossibility of any extraneous substance entering the area in thevicinity of the coil. It is thereby possible to stabilize theperformance of the exothermic resistor with respect to thermal orelectrical effects.

The above method of manufacturing the exothermic resistor isadvantageous because the sublimation of the core can be effectedsimultaneously with sintering of the glass member if the core wire 5 ismade of, for example, molybdenum. It is thus possible to remove the corewithout performing any special processing such as etching. Thiscontributes to simplification of the manufacturing operations.

Embodiment 8

Another example of the method of manufacturing the type of exothermicresistor shown in FIG. 3 will be described below.

A length of platinum wire 2 having a diameter of 20 μm is continuouslywound by an automatic winding machine around the core wire 5 made ofmolybdenum and having a diameter of 0.4 mm. A resistor blank therebyformed is cut into pieces each having a length of about 6 mm for oneexothermic resistor element or body. The lead wires 3 having a diameterof 0.13 mm and made of a platinum-iridium alloy are welded to oppositeends of each piece at the connections 21. By electrophoresis, the glassmaterial 4 is attached to outer surfaces of the molybdenum core wire 5and the platinum wire 2 except for opposite ends of the molybdenum core5. Thus-prepared blank piece is sintered in an oxidizing atmosphere bythe electric furnace. FIG. 5 shows the state of the blank piece afterthis sintering. In this case, the glass material 4 is, for example, aZnO--B₂ O₃ glass which has a viscosity of 10⁴ poise at 680° C. and whichcrystallizes at 750° C. and remelts at a temperature higher than about1000° C. As the temperature in the sintering process of the glassmaterial 4 rises, oxidation of the molybdenum core wire 5 is promoted sothat the core material becomes MOO₃. The softened glass is sealed at680° C. and crystallize at 750° C. so that the shape of the glass isstabilized. The sintering temperature is thereafter raised so that MoO₃is sublimated, thereby removing the molybdenum core 5. The sintering isthereafter continued at 950° C. for 20 minutes before it is finished.After the sintering has been completed, openings through whichsublimated MoO₃ is dispersed are left at opposite ends of the glassmember 4. These openings are closed by melting the glass by the heat ofa flame, thereby obtaining an exothermic resistor of the type shown inFIG. 3.

Embodiment 9

Still another example of the method of manufacturing the type ofexothermic resistor shown in FIG. 3 will be described below.

A length of platinum wire 2 having a diameter of 20 μm is wound by anautomatic winding machine around the core wire 5 made of molybdenum andhaving a diameter of 0.4 mm. A resistor blank thereby formed is cut intopieces each having a length of about 6 mm for one exothermic resistorelement or body. The lead wires 3 having a diameter of 0.13 mm and madeof a platinum-iridium alloy are welded to opposite ends of each piece atthe connections 21. A coating of the glass material 4 is formed byelectrophoresis. Thereafter, the thus-prepared blank piece is sinteredin an oxidizing atmosphere by the electric furnace. FIG. 3 shows thestate of the blank piece after this sintering. In this case, an Al₂ O₃--P₂ O₄ glass having a viscosity of 10⁶.7 poise at 820° C. and aviscosity of 10⁴ poise at 910° C. is selected as the glass material 4.As the temperature in the sintering process of this glass materialrises, the molybdenum core wire 5 is oxidized and sublimated at 795° C.so that it is removed. The baking is thereafter continued at 1080° C.for 1 hour before it is finished, thus obtaining an exothermic resistorof the type shown in FIG. 3.

As described above, the present invention was exemplified with respectvarious compositions of the glass material 4 shown in FIG. 3. If a glassmaterial has a viscosity higher than 10⁴ poise at 800° C. and aviscosity lower than 10⁷ poise at 1000° C., it can be used to form thetype of exothermic resistor shown in FIG. 3. Also, glass materials ofvarious compositions were tried with respect to the method in which theglass is formed as shown in FIG. 5. If a glass material is crystalizableat any temperature lower than 790° C. and capable of maintaining itsshape at a temperature lower than 900° C., it can be used to form thetype of exothermic resistor shown in FIG. 3.

In the above described embodiments, electrophoresis is utilized to formthe glass coating. However, methods other than the method of usingelectrophoresis, including a method of applying a glass material in theform of paste, are applicable to the manufacture of the exothermicresistor 1 shown in FIG. 3.

Embodiment 10

FIG. 6 shows the structure of an exothermic resistor which represents astill further embodiment of the present invention. Lead wires 3 made ofa platinum-iridium alloy are connected to opposite ends of a length ofexothermic resistance wire 2 in the form of a coil made of platinum. Theexothermic resistance wire 2, including connections 21, is covered witha layer of composite material 4 composed of ceramic and glass materialswhich constitute a support member.

A method of manufacturing this type of exothermic resistor will bedescribed below with reference to FIGS. 7A to 7E.

FIG. 7A shows a molybdenum core wire 5 having a diameter of 0.5 mm andprovided as the core around which the platinum wire is to be wound. Thecore wire 5 has circular column portions having a length of 5 mm andflat portions 5A having a length of 2 mm, the circular column portionsand the flat portions 5A being alternately disposed. As shown in FIG.7B, a length of platinum wire (exothermic resistor) 2 having a diameterof 30 μm was wound by an automatic winding machine around the core wire5 over a length for two or more resistor elements. As shown in FIG. 7C,a resistor blank thereby formed was cut at the centers of the flatportions into pieces, and a pair of lead wires 3 made of aplatinum-iridium alloy and having a diameter of 0.13 mm were welded toopposite ends of each piece at the connections 21. The flat portions 5Awere provided with a view to improving the facility with which the leadwires 3 was placed on and attached to the core wire 5. The flat portionswere formed by plastic working based on pressing. It is preferable interms of ease of working that the flat portions are made symmetricalabout a horizontal plane. As shown in FIG. 7D, the composite member 4was formed over the exothermic resistance wire 2 and is thereaftersintered. To apply a material to form this layer, a solution wasprepared which contained denatured alcohol and water as solvents andmagnesium nitrate and aluminum nitrate provided as electrolytes, and inwhich particles of alumina and powder of PbO--SiO₂ glass mixed at aratio: 95:5 were dispersed; and particles of the alumina and the glasswere attached to the platinum wire by electrophoresis in such a mannerthat the platinum wire in the state shown in FIG. 7C was placed in thesuspension for electrophoretic deposition as a cathode while an aluminumplate was placed in the same suspension as an anode, a voltage of 40 Vthen being applied through the suspension for electrophoretic depositionbetween these electrodes. FIG. 8 schematically illustrates this state ofcoating in which a glass-alumina layer 53 having pores 54 andconstituting a porous layer was coated to outer surfaces of the platinumwire 52 wound around the molybdenum core wire 51. As shown in FIG. 7D,the thus-prepared piece was heated in the electric furnace at 900° C.for 1 hour so that the molybdenum core wire was oxidized and sublimated,and this piece was heated at 1100° C. for 30 minutes, thereby sinteringthe electrocoating layer. The thickness of the sintered layer 4 wasabout 80 μm. The softening temperature of the glass used to form thislayer was 850° C., and the sintered layer 4 became porous but had astrength large enough to prevent itself from breaking during handling.As shown in FIG. 7E, this layer was coated with powder of PbO--B.sub. 2O₃ --SiO₂ glass having a softening temperature of 680° C. and thereafterunderwent baking at 850° C. for 90 minutes so that this glass permeatedthrough the sintered layer 4, thereby forming a composite material layer4A. From observation of a cross-section of the thus-obtained exothermicresistor, it was found that the coating glass formed a surface layer andalso reached the cavity formed by the removal of the molybdenum corewire, thereby forming a continuous phase. The volume percent of theglass contained in the composite material layer was 32%, and the forcerequired to crush the thus-obtained exothermic resistor is 2.1 kg whilethe anti-crushing strength of a resistor manufactured by theconventional technique is about 0.5 kg at most.

Embodiment 11

A wire having a diameter of 0.5 mm and made of a Ni-Fe alloy having a53% nickel content was used as the core wire. In the same process asthat shown in FIGS. 7A to 7E, a platinum wire was wound around this corewire, a resistor blank thereby formed around the core wire was cut intopieces, and lead wires were welded to each cut piece. Alumina particleswere coated to the welded portions and to the platinum wire by a dippingmethod. This method resides in a process in which a solution is preparedby dispersing alumina particles in an organic solvent (terpineol); andthe exothermic resistor blank constituted by the core, the platinum wirewound around the core and the lead wires connected to the platinum wireis dipped in this solution and is taken out therefrom, thereby coatingalumina particles to the exothermic resistor. In this case, one end ofthe core was not coated with alumina particles. Each blank piece washeated at 1500° C. for 2 minutes, thereby sintering the alumina. Theblank piece was thereafter immersed in a mixed acid composed of nitricacid and sulfuric acid at 80° C. for 3 hours, thereby removing the coreby the etching manner. The resistor piece was thereafter coated withpowder of PbO-SiO₂ glass having a softening temperature of 600° C. andthereafter underwent baking at 820° C. for 90 minutes so that this glasspermeated through the baked alumina layer, thereby forming a compositematerial layer. The anti-crushing strength of the thus-obtainedexothermic resistor was 1.8 kg, and the volume percent of the glass inthe composite material layer was 41%.

Embodiment 12

Exothermic resistors similar to those of Embodiments 10 and 11 andhaving glass components of different volume percents in the compositematerial layers were manufactured. Of these exothermic resistors, onehaving a glass component of a smaller volume percent was manufactured insuch a manner that ceramic particles and glass particles weresimultaneously coated, at a desired mixing ratio, to the platinum wireby electrophoresis in the same manner as in the case of Embodiment 10and were sintered under conditions for enabling the glass to suitablymelt. The exothermic resistor was completed without performing thesucceeding glass coating. To form each of exothermic resistors of thistype, composite material particles were preliminarily formed by mixingceramic particles and glass particles at a desired ratio, heating thismixture so that the glass was molten, cooling to solidify the same, andpulverizing the composite material thereby obtained; and the compositematerial particles thereby formed were coated to the platinum wire,thereby improving the uniformity of the materials constituting thecomposite material layer. Since, in the case of attachment of particlesbased on the electrophoresis method, action of electric charges on thesurfaces of particles is utilized, the manner of attachment variesdepending upon the type of particle. For this reason, the ceramic andthe glass are not always attached while being maintained at the samecompounding ratio as that at which they are originally mixed, and thereis therefore a possibility of occurrence of non-uniformity of thecompounding ratio with respect to the area over which the mixedparticles are attached. However, it is possible to avoid this problem bypreparing composite material particles each of which contains theceramic and glass materials mixed at a predetermined compounding ratio.Silicon carbide, silicon nitride and aluminum nitride were also used asceramic components other than the alumina. Thermal conductivities ofalumina, silicon carbide, silicon nitride, and aluminum nitride are 21W/m·K, 40 W/m·K, 12 W/m·K, and 21 W/m·K, respectively. When thesematerials other than the alumina were used, sintering after the coatingof the composite material was performed in an inert gas.

The anti-crushing strength and the response time were examined withrespect to exothermic resistors thereby manufactured. FIG. 9 shows therelationship between the ratio of the volume of glass component to thevolume of the composite material layer (volume percent) and theanti-crushing strength, the abscissa representing the volume percent Rand the ordinate representing the anti-crushing strength F (kg). Thebroken line F₀ indicates a level of anti-crushing strength required foran ordinary exothermic resistor, and the anti-crushing strengths of theexothermic resistors manufactured in accordance with the presentinvention fall into a region between the solid lines O and D. Theanti-crushing strength varies over a certain range depending uponfactors including the type of ceramic component of the compositematerial. FIG. 10 shows the relationship between the volume percent ofthe glass component and the response time, the abscissa representing thevolume percent R and the ordinate representing the response time T (ms).The response time also varies depending upon factors including the typeof ceramic component of the composite material.

When the volume percent of the glass component of the composite materiallayer was less than 2%, the strength of the layer was so small that itwas impossible to handle the resistor piece by employing a pincette orthe like. After the composite material had been sintered at a hightemperature for a long time in order to increase the strength, thecharacteristics of the platinum wire changed, which impaired the desiredcharacteristics of the exothermic resistor.

When the volume percent of the ceramic component of the compositematerial layer was less than 40%, that is, the volume percent of theglass component was higher than 60%, the response characteristics of theresulting air flow meter became deteriorated as in the case where theplatinum wire was covered with glass alone. As a results, effectsspecific to the composition of the ceramic and glass materials were notobtained.

Embodiment 13

In the case of Embodiments 10 to 12, the wire to be used as anexothermic resistance element was continuously wound around the corewire, the resistor blank thereby formed was cut into pieces each havinga desired length, and the lead wires were connected to each piece. Inthis embodiment, however, the core wire is cut into pieces each having adesired length; a pair of lead wires are connected to opposite ends ofeach cut core wire; one end of a wire to be used as an exothermicresistance element was connected to one of the pair of lead wires; andthe other end of the resistance wire is connected to the other lead wireafter the resistance wire has been wound around the core wire. FIGS. 11Ato 11D show procedures of manufacturing an exothermic resistor in thismanner. As shown in FIG. 11A, a pair of lead wires having a diameter of0.13 mm and made of a platinum-iridium alloy are connected to flatportions of a molybdenum core wire 5 having a diameter of 0.5 mm, theflat portions being formed at opposite ends thereof. As shown in FIG.11B, one end a platinum wire (exothermic resistor) 2 having a diameterof 30 μm is welded to one of the pair of lead wires 3 at a connection21, and the other end of the platinum wire 2 is welded to the other leadwire 3 at another connection 21 after it has been wound around the corewire 5. As shown in FIG. 11C, a composite material is applied over theexothermic resistance wire 2 and is baked. As shown in FIG. 11D, theresistor piece is coated with glass powder and thereafter undergoessintering so that the glass permeates through the sintered layer 4,thereby forming a composite material layer 4A.

Embodiment 14

A type of exothermic resistor formed on an alumina substrate will bedescribed below with reference to FIGS. 12A to 12D and FIG. 13. Acircuit 94 was formed by a lift-off method on an alumina substrate 91having a width of 4 mm, a length of 10 mm, and a thickness of 0.3 mm.FIG. 12A to 12D show main procedures based on the lift-off method byillustrating cross-sectional views of the exothermic resistor; FIG. 12Ashows a step of forming a mask 92 on the substrate 91 from aphotoresist; FIG. 12B, a step of forming a film over the substrate andthe mask by applying a platinum paste 93 thereto; FIG. 12C, a step ofcutting the film by dilating the resist by a developer; and FIG. 12D, astep of removing the mask 92 by an etching manner and thereaftercompleting the platinum circuit 94 by baking. The pattern of theplatinum film circuit was formed while the width of circuit lines wasset to 400 μm and the distance between the lines was set to 100 μm. Theresistance of this circuit was 12 Ω. The pattern had at its oppositeends portions of large areas to which a pair of lead wires 95 made of aplatinum-iridium alloy were connected by brazing. A paste containing aPbO-SiO₂ glass having a softening temperature of 600° C. and alumina,the ratio of the proportions of the glass and the alumina being 2:3, wasapplied over the platinum thick film circuit and was sintered at 800° C.for 15 minutes, thereby forming a composite material layer 96. Theresponse speed of an air flow meter constituted by employing thethus-obtained exothermic resistor 100 was twice as high as that in thecase there the circuit was coated with glass alone.

FIG. 14 shows a hot wire air flow meter which makes use of theexothermic resistor 1 of the present invention. In this air flow meter,a resistor 6 for measuring the temperature of air which is the same asthe exothermic resistor 1 is used in combination with the exothermicresistor 1. As shown in FIG. 14, the exothermic resistor 1 and theresistor 6 for measuring the temperature of air are secured to a support8 disposed in a bypass passage 72 which is formed in a body 73 and whichbypasses part of intake air the majority of which flows through a mainpassage 71 also formed in the body 73.

FIG. 15 shows a circuit for driving the hot wire air flow meter,constituted by the exothermic resistor 1, the air temperaturemeasurement resistor 6, operational amplifiers 9 and 10, a powertransistor 11, capacitor 12, and resistors 13 to 17. The plus terminalof a battery (not shown) is connected to the collector terminal 18 ofthe power transistor 11 while the minus terminal of the battery isconnected to a grounding terminal 19 of the resistor 13. An inputterminal of a microcomputer (not shown) for controlling an engine byusing signals output from the hot wire air flow meter is connected to aconnection 20 between the resistor 13 and the exothermic resistor 1.

In the thus-constructed circuit, an electric current is supplied to theexothermic resistor 1 by the power transistor 11 in order to heat up theexothermic resistor, and the temperature of the exothermic resistor iscontrolled in such a manner that it is kept higher than that of the airtemperature measurement resistor 6 by constant degrees. During thiscontrol, the air temperature measurement resistor 6 is used to correctthe temperature of the intake air by detecting this temperature whileallowing only a very weak current to flow through the air temperaturemeasurement resistor 6 such that the heat generated by this current isnegligible. As the air flows while colliding with the exothermicresistor 1, the driving circuit performs the control operation toconstantly maintain the difference between the temperatures of theexothermic resistor 1 and the air temperature measurement resistor 6, asdescribed above. This operation is performed in a feedback manner suchthat a voltage obtained by dividing the voltage across the exothermicresistor 1 by means of the resistors 14 and 15 is constantly kept equalto a voltage amplified by the operational amplifier 9 from a voltagedrop across the resistor 13 proportional to the current flowing throughthe exothermic resistor 1. In consequence, as the air flow rate changes,the current flowing through the exothermic resistor 1 changes, and theair flow rate is measured from the voltage drop that appears across theresistor 13 in response to the current.

FIG. 16 shows a graph of response characteristics of this hot wire airflow meter in accordance with the present invention. The abscissarepresents the time (ms) and the ordinate represents the flow rate(kg/h). The voltage output from the hot wire air flow meter was measuredwhen the air flow rate was changed from a low flow rate of about 20 kg/hto a high flow rate of about 200 kg/h. This voltage was converted intothe flow rate to be plotted along the ordinate. The curve B indicates acharacteristic of the air flow meter making use of the conventionalbobbin-type exothermic resistor, and the curve A indicates acharacteristic of the air flow meter in accordance with the presentinvention in comparison with the former. As can be understood from thisgraph, the present invention ensures that the time taken for the flowmeter to output the final value can be remarkably reduced.

It is therefore possible for the hot wire air flow meter to outputsignals correctly in response to actual changes in the air flow rateeven at the time of rapid acceleration or deceleration of the vehicle,thereby optimizing the determination of the injection rate of theinjector and solving the problem of surging.

This remarkable improvement in the response performance is attained forthe reason that the exothermic resistor 1 can rapidly react to a changein the air flow rate since the heat heat generating in the platinum wire2 of the exothermic resistor 1 is almost entirely transmitted to the airwithout heating the bobbin or core or being transmitted to the supportvia the bobbin as in the case of the conventional bobbin-type exothermicresistor.

A type of anemometer was also manufactured which was designed to utilizea combination of the exothermic resistor of the present invention and acircuit for detecting the temperature from a change in the resistancevalue of this resistor and converting it into the wind velocity. As aresult, this anemometer also exhibited improved responsecharacteristics.

What is claimed is:
 1. A hot wire air flow meter comprising: anexothermic resistor disposed in an air flow passage, said exothermicresistor being used to measure the rate at which air flows through saidair flow passage; and a driving circuit for controlling the currentflowing through said exothermic resistor and taking out a voltage outputfrom said exothermic resistor as a signal corresponding to said air flowrate, said exothermic resistor having a coiled wire serving as anexothermic resistance wire, a pair of lead wires connected to saidcoiled wire, and a support member covering and supporting said coiledwire, wherein said support member is formed of a layer of a compositematerial composed of ceramic and glass materials.
 2. A hot wire air flowmeter according to claim 1, wherein a cavity is formed in saidexothermic resistor, and the glass component of the composite materialforming said support member forms a surface layer of said supportmember, said glass component forming a continuous phase reaching saidcavity.
 3. A hot wire air flow meter according to claim 1, the ceramiccomponent of said composite material forming said support member has athermal conductivity of at least 10 W/m·K.
 4. A hot wire air flow meteraccording to claim 1, the proportion of the glass component of saidcomposite material forming said support member is 2 to 60 volumepercent.
 5. A hot wire air flow meter according to claim 1, saidcomposite material forming said support member contains a glasscomponent having a softening temperature not higher than 700° C. andanother glass component having a softening temperature higher than 700°C.
 6. An air flow meter comprising:an exothermic resistor disposed in anair flow passage, means for measuring an output of the exothermicresistor to detect air flow in the air flow passage, and means forcontrolling the rate of air flow in the air flow passage in response tothe output; wherein said exothermic resistor comprises a porous ceramicbody having a hollow cavity, with internal surfaces of the ceramic bodydefining the cavity, and a filling material filling the porosity of theceramic body, a coiled wire formed on the internal surfaces of theceramic body, and a pair of terminals extending outwardly from theceramic body and being electrically connected to the coiled wire.
 7. Theair flow meter according to claim 6, further comprising a secondresistor provided in the air flow passage, and wherein the exothermicresistor is provided at an upstream side of the air flow passagerelative to positioning of the second resistor.
 8. The air flow meteraccording to claim 7, further comprising means for defining a bypasspassage of the air flow passage, said exothermic resistor and saidsecond resistor being provided in said bypass passage.
 9. The air flowmeter according to claim 6, wherein the ceramic body has a thermalconductivity of at least 10 W/m·K.
 10. The air flow meter according toclaim 6, wherein the filling material comprises a glass component. 11.The air flow meter according to claim 6, wherein a proportion of thefilling material filling the porosity of the ceramic body is 2 to 60percent by volume.
 12. The air flow meter according to claim 6, whereinthe filling material filling the porosity of the ceramic body contains aglass component having a softening temperature not higher than 700° C.and another glass component having a softening temperature higher than700° C.
 13. An internal combustion engine comprising the air flow meteras defined in claim 6, provided in an intake air passage of the internalcombustion engine.
 14. The internal combustion engine according to claim13, wherein the air flow meter comprises a glass component provided insaid cavity so as to support said coiled wire, said glass componenthaving a viscosity of 10⁴ to 10⁷ poise at a temperature of 800° to 850°C.
 15. The internal combustion engine according to claim 13, whereinsaid glass component is provided in said cavity so as to support saidcoiled wire and connections between said coiled wire and said terminals.16. The internal combustion engine according to claim 13, wherein theair flow meter comprises a glass component provided in said cavity so asto support said coiled wire, said glass component being selected fromthe group consisting of lead-potash glass, lead-soda glass andlead-potash-soda glass essentially consisting, by weight, of 50 to 65%of SiO₂, 20 to 35% of PbO, and 10 to 20% of R₂ O, R₂ O being the sum ofK₂ O and Na₂ O.
 17. An air flow meter comprising:an exothermic resistordisposed in an air flow passage, means for measuring an output of theexothermic resistor to detect air flow in the air flow passage, andmeans for controlling the rate of air flow in the air flow passage inresponse to the output; wherein said exothermic resistor has a coiledwire serving as an exothermic resistance wire, a pair of lead wiresconnected to said coiled wire, and a support member covering andsupporting said coiled wire, wherein said support member is formed of alayer of a composite material composed of ceramic and glass materials.18. A hot wire air flow meter comprising: an exothermic resistordisposed in an air flow passage, said exothermic resistor being used tomeasure the rate at which air flows through said air flow passage; and adriving circuit for controlling the current flowing through saidexothermic resistor and taking out a voltage output from said exothermicresistor as a signal corresponding to said air flow rate, saidexothermic resistor having a coiled wire, a pair of lead wires connectedto opposite ends of said coiled wire, connections between said coiledwire and said lead wires, and a support member, which is a singleintegral member, for integrally supporting said connections and saidcoiled wire located between said connections, said support membercovering the coiled wire located between the connections, and theconnections, such that the coiled wire located between the connections,and said connections are fixed.
 19. A hot wire air flow metercomprising: an exothermic resistor disposed in an air flow passage, saidexothermic resistor being used to measure the rate at which air flowsthrough said air flow passage; and a driving circuit for controlling thecurrent flowing through said exothermic resistor and taking out avoltage output from said exothermic resistor as a signal correspondingto said air flow rate, said exothermic resistor having a coiled wire, apair of lead wires connected to opposite ends of said coiled wire,connections between said coiled wire and said lead wires, and a supportmember, which is a single integral member, for integrally supportingsaid connections and said coiled wire located between said connections,the support member covering said connections and the coiled wire locatedbetween the connections.
 20. A hot wire air flow meter according toclaim 19, wherein said support member is formed from a glass having aviscosity of 10⁴ to 10⁷ poise at temperatures of 800° to 850° C.
 21. Ahot wire air flow meter according to claim 19, wherein said supportmember is formed from lead-potash glass, lead-soda glass orlead-potash-soda glass essentially consisting, by weight, 50 to 65% ofSiO₂, 20 to 35% of PbO, and 10 to 25% of R₂ O (R₂ O is the sum of K₂ Oand Na₂ O).
 22. A hot wire air flow meter according to claim 19, whereinsaid support member is formed from soda-lime glass, soda-barium glass,potash-lime glass or potash-barium glass essentially consisting, byweight, 65 to 75% of SiO₂, 4 to 20% of R₂ O (R₂ O is the sum of K₂ O andNa₂ O).
 23. A hot wire air flow meter according to claim 19, whereinsaid support member is formed from a borosilicate glass.
 24. Anexothermic resistor for use in a hot wire air flow meter, comprising acoiled wire, a pair of lead wires connected to opposite ends of saidcoiled wire, connections between said coiled wire and said lead wires,and a support member, which is a single integral member, for integrallysupporting said connections and said coiled wire located between saidconnections, the support member covering the coiled wire located betweenthe connections, and said connections, such that the coiled wire locatedbetween the connections, and said connections, are fixed.
 25. Anexothermic resistor for use in a hot wire air flow meter, comprising acoiled wire, a pair of lead wires connected to opposite ends of saidcoiled wire, connections between said coiled wire and said lead wires,and a support member, which is a single integral member, for integrallysupporting said connections and said coiled wire located between saidconnections, the support member covering said connections and the coiledwire located between the connections.
 26. An exothermic resistoraccording to claim 25, wherein said support member is formed from aglass having a viscosity of 10⁴ to 10⁷ poise at temperatures of 800° to850° C.
 27. An exothermic resistor according to claim 25, wherein saidsupport member is formed from lead-potash glass, lead-soda glass orlead-potash-soda glass essentially consisting, by weight, 50 to 65% ofSiO₂, 20 to 35% of PbO, and 10 to 20% of R₂ O (R₂ O is the sum of K₂ Oand Na₂ O).
 28. An exothermic resistor according to claim 25, whereinsaid support member is formed from soda-lime glass, soda-barium glass,potash-lime glass or potash-barium glass essentially consisting, byweight, 65 to 75% of SiO₂, 4 to 15% of RO (RO is the sum of MgO, CaO andBaO), and 10 to 20% of R₂ O (R₂ O is the sum of K₂ O and Na₂ O).
 29. Anexothermic resistor according to claim 25, wherein said support memberis formed from a borosilicate glass.
 30. An air flow meter comprising:anexothermic resistor disposed in an air flow passage, means for measuringan output of the exothermic resistor to detect air flow in the air flowpassage, and means for controlling the rate of air flow in the air flowpassage in response to the output; wherein said exothermic resistor hasa coiled wire, a pair of lead wires connected to opposite ends of saidcoiled wire, with connections between said coiled wire and said leadwires, and a support member, which is a single integral member, forintegrally supporting said connections and said coiled wire locatedbetween said connections, the support member covering the coiled wirelocated between the connections, and the connections, such that thecoiled wire located between the connections, and the connections, arefixed.
 31. An air flow meter comprising:an exothermic resistor disposedin an air flow passage, means for measuring an output of the exothermicresistor to detect air flow in the air flow passage, and means forcontrolling the rate of air flow in the air flow passage in response tothe output; wherein said exothermic resistor has a coiled wire, a pairof lead wires connected to opposite ends of said coiled wire, withconnections between said coiled wire and said lead wires, and a supportmember, which is a single integral member, for integrally supportingsaid connections and said coiled wire located between said connections,the support member covering the connections and the coiled wire locatedbetween the connections.